Sampling oscilloscopes were developed more than twenty years ago to respond to small, fast-changing signals that conventional oscilloscopes could not respond to because of limited bandwidth or risetime characteristics. Sampling is a now well-known technique in which the signal path is gated for an extremely short period of time to pass the substantially instantaneous amplitude value (voltage sample) of the electrical signal during that period. Each sample taken in this manner is processed by electronic circuits and displayed at an appropriate respective time position. Since the samples appear on a cathode-ray tube (CRT) display as dots, a large number of samples are required to reconstruct a waveform. Generally speaking, sampling is practical primarily when the electrical signal is repetitive in nature since in most cases it is impossible or impractical to acquire all of the needed samples during a single event or single cycle of the signal. Indeed, one of the practical advantages of sampling is that at least one sample can be acquired from each of a large number of cycles, and a representative waveform may be reconstructed and displayed therefrom.
Sampling modes are typified in accordance with the timing method used. Sequential sampling is a mode in which the display is comprised of a very orderly series of equally spaced dots. Random sampling is a mode in which successive dots may occur at what appears to be random horizontal positions because the sampling timing and signal triggering are unrelated, although it must be pointed out that with random sampling the reconstructed waveform is defined because the dots are inserted into the display at substantially correct time positions. Real-time sampling is a mode in which the electrical signal is very slow, allowing all of the samples to be acquired on one cycle so that the waveform displayed is the one actually sampled.
Prior art sampling systems, referred to herein as classic samplers, include a high-speed sampling gate, a sampling capacitor, a memory gate, a memory capacitor, associated amplifiers including a sampling preamplifier and a memory amplifier, and a feedback attenuator from the memory amplifier output to the sampling preamplifier input. A sampling loop is formed having a forward gain from input to output and a feedback attenuation factor from output to input to establish a loop gain which is ideally unity. The purpose of the feedback attenuator is to place a voltage on the sampling capacitor which is equal (for unity loop gain) to the last sample taken and an estimator of the next sample to be taken. With each sample, the memory output repeatedly attempts to reduce to zero the voltage existing between the input and output of the sampling gate. If the input voltage is the same each time it is sampled, the feedback matches it, reducing the difference and the size of each amplified step to practically zero. The ability of a sampling oscilloscope to display correctly the voltage change between any two successive samples is known as dot transient response, and good dot transient response requires unity loop gain.
There are situations in which it is desirous to have a loop gain which is not equal to one. For example, in a process known as smoothing, i.e., reducing the effect of random noise or jitter in the display, the loop gain is set to some value less than one. However, smoothing results in a degraded dot transient response since the estimator placed on the sampling capacitor is derived from the previous sample, may be different from the new sample about to be taken. The viewed reconstructed waveform has the appearance of having been passed through a low-pass filter, and while noise is reduced on the displayed waveform, the leading edge of a square-wave pulse is rolled off. Moreover, signal noise is not completely eliminated, and in certain situations appears smeared along the displayed waveform. Smoothing with random sampling is impossible because of the unpredictability of where a sample is taken.
Other problems associated with prior art classical samplers include distortion and inaccuracies due to the sampling loop always being open to some extent, difficulties in determining what loop gain actually is, memory "droop" due to capacitor leakage, and complexities in building and making accurate portions of the analog system because even component lead lengths are often critical.